1. Field of the Invention
The present invention relates to a semiconductor device provided with a function to control a current supply to a load by a transistor. In particular, the invention relates to a semiconductor device including a pixel formed of a current driving light emitting element of which luminance changes according to current, and a signal driver circuit thereof.
2. Description of the Related Art
In recent years, what is called a self-light emitting display device of which pixel is formed of a light emitting element such as a light emitting diode (LED) is attracting attentions. As a light emitting element used for such a self-light emitting display device, an organic light emitting diode (OLED), an organic EL element, an electroluminescence (EL) element and the like are attracting attentions and used for an organic EL display and the like.
A light emitting element such as an OLED which is a self-light emitting element is advantageous as compared to a liquid crystal display in that a pixel provides a high visibility, a backlight is not required, a high response speed is achieved, and the like. The luminance of a light emitting element is controlled by a current value supplied thereto.
As a driving method of a display device using such a self-light emitting element, a passive matrix method and an active matrix method are known. With the passive matrix method, a structure is simple but a large display of high luminance is difficult to achieve. The active matrix method in which a current supplied to a light emitting element is controlled by a thin film transistor (TFT) provided in a pixel circuit is more actively developed recently.
In the case of such an active matrix display device, there is a problem in that a luminance changes when a current supplied to a light emitting element changes due to variations in current characteristics of driving TFTs.
In other words, in the case of such an active matrix display device, driving TFTs which drive a current supplied to a light emitting element is used in a pixel circuit. When the characteristics of these driving TFTs vary, a current supplied to a light emitting element changes, leading to vary the luminance. In view of this, various circuits for suppressing variations in luminance have been suggested, in which a current supplied to a light emitting element does not change even when characteristics of driving TFTs in a pixel circuit vary.
[Patent Document 1]    Published Japanese Translation of PCT International Publication for Patent Application No. 2002-517806
[Patent Document 2]    International Publication WO01/06484
[Patent Document 3]    Published Japanese Translation of PCT International Publication for Patent Application No. 2002-514320
[Patent Document 4]    International Publication WO02/39420
Patent Documents 1 to 4 each discloses a structure of an active matrix display device. Patent Documents 1 to 3 each discloses a circuit configuration in which a current supplied to a light emitting element does not change due to variations in characteristics of driving TFTs provided in a pixel circuit. This structure is referred to as a current write type pixel, a current input type pixel or the like. Patent Document 4 discloses a circuit configuration for suppressing a change of signal current due to variations of TFTs in a source driver circuit.
FIG. 31 shows a first schematic example of a conventional active matrix display device disclosed in Patent Document 1. The pixel in FIG. 31 includes a source signal line 3101, first to third gate signal lines 3102 to 3104, a current supply line 3105, TFTs 3106 to 3109, a capacitor 3110, an EL element 3111, a signal current input current source 3112.
A gate electrode of the TFT 3106 is connected to the first gate signal line 3102, a first electrode thereof is connected to the source signal line 3101, a second electrode thereof is connected to a first electrode of the TFT 3107, a first electrode of the TFT 3108, and a first electrode of the TFT 3109. A gate electrode of the TFT 3107 is connected to a second gate signal line 3103, and a second electrode thereof is connected to a gate electrode of the TFT 3108. A second electrode of the TFT 3108 is connected to a current supply line 3105. A gate electrode of the TFT 3109 is connected to a third gate signal line 3104 and a second electrode thereof is connected to an anode of the EL element 3111. The capacitor 3110 is connected between the gate electrode and an input electrode of the TFT 3108 and holds a gate-source voltage of the TFT 3108. The current supply line 3105 and a cathode of the EL element 3111 are inputted with predetermined potentials respectively and have a potential difference therebetween.
An operation from writing of a signal current to light emission is described with reference to FIGS. 32A to 32E. In FIGS. 32A to 32E, reference numerals which denote each portion are referred in FIG. 31. FIGS. 32A to 32C each shows a current flow schematically. FIG. 32D shows a relationship of a current flowing through each path when writing a signal current. FIG. 32E shows a gate-source voltage of the TFT 3108, which is a voltage accumulated in the capacitor 3110 when writing a signal current.
First, a pulse is inputted to the first gate signal line 3102 and the second gate signal line 3103, thereby the TFTs 3106 and 3107 are turned on. At this time a current flowing through a source signal line, which is a signal current is called Idata.
As a current Idata flows through a source signal line, a current path is divided into I1 and I2 in a pixel as shown in FIG. 32A. The relationship between these is shown in FIG. 32D. It is to be noted that Idata=I1+I2 is satisfied, needless to say.
A charge is not held in the capacitor 3110 just after the TFT 3106 is turned on, therefore, the TFT 3108 is off. Accordingly, I2=0 is satisfied and Idata=I1 is satisfied. That is to say, only a current accumulated in the capacitor 3110 flows at this time.
After that, a charge is accumulated in the capacitor 3110 gradually, which generates a potential difference between both electrodes (FIG. 32E). When the potential difference between the both electrodes becomes Vth (point A in FIG. 32E), the TFT 3108 is turned on and I2 generates. As described above, as Idata=I1+I2 is satisfied, I1 decreases gradually, however, a current still flows and a charge keeps being accumulated in the capacitor.
In the capacitor 3110, a charge keeps being accumulated until the potential difference between the both electrodes, which is a gate-source voltage of the TFT 3108 reaches a desired voltage, which is a voltage (VGS) high enough for the TFT 3108 to flow a current Idata. When the charge accumulation is terminated (point B in FIG. 32E), the current I1 stops flowing and a current corresponding to VGS at that time flows through the TFT 3108, leading to satisfy Idata=I2 (FIG. 32B). In this manner, a steady state is achieved. The writing operation of a signal is completed in this manner. At last, selection of the first gate signal line 3102 and the second gate signal line 3103 is terminated, which turns off the TFTs 3106 and 3107.
Subsequently, a light emitting operation starts. A pulse is inputted to the third gate signal line 3104, thereby the TFT 3109 is turned on. As VGS which is just written is held in the capacitor 3110, the TFT 3108 is on and a current Idata flows from the current supply line 3105 to the EL element 3111. Accordingly, the EL element 3111 emits light. At this time, by setting the TFT 3108 to operate in the saturation region, Idata can flow without change even when a drain-source voltage of the TFT 3108 changes.
An operation to output a set current in this manner is called an output operation. The current write type pixel of which example is shown above is advantageous in that a desired current can be accurately supplied to an EL element since a gate-source voltage required to flow the current Idata is held in the capacitor 3110 even when characteristics and the like of the TFT 3108 vary. Accordingly, variations in luminance due to variations in characteristics of TFTs can be suppressed.
The aforementioned example relates to a technique for correcting a change of current due to variations of driving TFTs in a pixel circuit. A similar problem occurs in a source driver circuit as well. Patent Document 4 discloses a circuit configuration for suppressing a change of signal current due to variations of TFTs in a source driver circuit.